Valve Mechanism and Mechanical Lash Adjuster

ABSTRACT

A valve mechanism is provided with a cam, a valve urged in a valve closing direction by a valve spring, a rocker arm, and a lash adjuster. The lash adjuster may include a plunger to which a pressing force of the cam and an urging force of the valve spring are transmitted through the rocker arm, a housing that forms a thread engagement portion, and a compression coil spring urging the plunger in the extension direction. The thread engagement portion may be such that a load acting on the plunger in the plunger extension or contraction direction causes sliding rotation in the thread engagement portion to be suppressed, and such that a lateral load acting on the plunger causes the suppression of sliding rotation to be relieved.

CROSS-REFERENCE TO RELATED APPLICATION, PRIORITY CLAIM, ANDINCORPORATION BY REFERENCE

This application is the national stage of International Application No.PCT/JP2017/022123, entitled “Valve Mechanism and Mechanical LashAdjuster”, filed 15 Jun. 2017; and claims benefit of priority under 35USC 119(a)-(d) and 35 USC 365(b) to International Application No.PCT/JP2016/068045, entitled “Mechanical Lash Adjuster”, filed 17 Jun.2016, the contents of both of which applications are incorporated hereinin their entireties by reference.

TECHNICAL FIELD

The present invention relates to a valve mechanism automaticallyadjusting a valve clearance (e.g., a gap between a cam and a rocker armin a rocker-arm valve mechanism or a gap between a cam and a tappet(bucket) covering a stem in a direct-acting valve mechanism) and amechanical lash adjuster used in the valve mechanism.

BACKGROUND ART

It is widely known that when an intake valve or an exhaust valve used inan engine of an automobile etc. is mounted on an intake port or anexhaust port of a cylinder head, for example, a rocker arm linked to astem is configured to swing by using a mechanical lash adjuster as afulcrum, so as to automatically adjust a valve clearance through driving(extension/contraction motion) of the mechanical lash adjuster (e.g.,see Patent Documents 1, 2, and Non-Patent Literature 1).

This type of the mechanical lash adjuster includes a plunger (pivotmember) having a male thread formed on the outside and a cylindricalhousing that is a plunger engaging member having a female thread formedon the inside, and has a structure in which the male thread on theoutside of the plunger is screwed into the female thread on the insideof the housing to form a thread engagement portion and a plunger spring(compression coil spring) is housed in the housing such that the plungerspring urges the plunger toward a rocker arm on the upper side. Bysetting angles (lead and flank angles) of “thread ridges” of “buttressthreads” made up of the female thread on the housing side and the malethread on the plunger side to predetermined angles, the plunger isallowed to slide and rotate in the thread engagement portion and therebymoved in a direction in which the plunger projects from the housing(hereinafter referred to as a “plunger extension direction”) under anaxial load in the same direction, while the slide rotation of theplunger is suppressed in the thread engagement portion (hereinafter,this will be referred to as “threads” being made self-sustaining) by afriction generated in the thread engagement portion in a direction inwhich the plunger sinks into the housing (hereinafter referred to as a“plunger contraction direction”) under an axial load in the samedirection, and the valve clearance is thereby automatically adjusted.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Unexamined Patent Application Publication(Translation of PCT Application) No. 61-502553 (FIGS. 1 to 5)

Patent Document 2: Japanese Unexamined Utility Model ApplicationPublication No. 3-1203 (FIGS. 1 to 3)

Patent Document 3: WO2013-136508A

Non-Patent Literature

Nonpatent Literature 1: NTN TECHNICAL REVIEW No. 75 (2007), Article“Development of the End-Pivot Type Mechanical Lash Adjuster” (pp. 78-85,FIGS. 1 to 4)

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

However, while the conventional mechanical lash adjusters (PatentDocuments 1, 2, and Non-Patent Document 1) can operate in the directionof reducing the valve clearance (the plunger extension direction) whenthe valve clearance is increased, the mechanical lash adjusters have noadjust structure actively increasing the valve clearance (adjusting thevalve clearance to zero) in the operation in the direction of increasingthe valve clearance (the plunger contraction direction) when the valveclearance is reduced, although having a margin for adjustment of threadbacklash (backlash).

Therefore, for example, if the engine is rapidly cooled after beingstopped in a warmed state, the valve clearance may be put into anexcessively small (negative clearance) state due to a difference inthermal expansion coefficient between a cylinder head (aluminum alloy)and a valve (iron alloy) so that a face surface of the valve may floatfrom a valve seat, and in such a situation, since the conventional lashadjusters cannot operate in the plunger contraction direction (directionof increasing the valve clearance), the excessively small (negativeclearance) state of the valve clearance is left as it is, leading to arisk of an excessing valve lift amount or defective sealing between theface surface of the valve and the valve seat (sealing of a combustionchamber) when the engine is restarted while being cold.

Therefore, in consideration of the problem described above, as describedin Patent Document 3, the present inventors made a proposition that bysetting a lead angle and a flank angle of thread ridges of “threads”constituting a thread engagement portion (e.g., setting the lead angleand the flank angle to ranges of 10 to 40 degrees and 5 to 45 degrees,respectively), the plunger is allowed to slide and rotate in the threadengagement portion and moved in an axial-load acting direction when anaxial load acts on the plunger in either of extension and contractiondirections and, if the sum of friction torques respectively generated onsliding contact surfaces of the plunger with an axial-load transmittingmember (such as a rocker arm) and the plunger spring exceeds a thrusttorque causing the plunger to slide and rotate in the thread engagementportion, the threads of the thread engagement portion are madeself-sustaining (the slide rotation of the plunger is suppressed in thethread engagement portion and the plunger is made immovable in thethread engagement portion).

However, the present inventors continued experiments to find that whenthe mechanical lash adjuster according to Patent Document 3 is used, thefollowing new problem occurs although the problem described above issolved.

Specifically, in the excessively small state of valve clearancegenerated if the engine is rapidly cooled after being stopped in awarmed state or the valve seat surface is worn out, when it is supposedthat the plunger should sink so as to eliminate the excessively smallstate of valve clearance by a proper amount to a predetermined positionat which the sum of friction torques respectively generated on thesliding contact surfaces of the plunger with the axial-load transmittingmember (such as a rocker arm) and the plunger spring exceeds the thrusttorque causing the plunger to slide and rotate in the thread engagementportion, the plunger sinks more than the proper amount, causing anunexpected state (new problem) in which a ramp portion (a portionadjusting acceleration of a valve) between a base circle and a cam noseof a cam fails to function, resulting in a hitting noise of the cam nosehitting the axial-load transmitting member or a collision noise of aface surface (seat) of a head colliding with a valve seat insert.

As a result of studies by the present inventors on the cause thereof, itwas found that while a backlash (clearance between male and femalethreads) is always provided between the male and female threadsconstituting the thread engagement portion, this backlash is the causeof the “excessive sinking amount of the plunger”.

Specifically, for example, in a rocker-arm valve mechanism in which apressing force of a cam acts on a plunger via a rocker arm, when acontact point between the cam and the rocker arm moves on the rockerarm, in addition to an axial load along the axis of the plunger, alateral load (see reference numerals T1, T2 of FIG. 5) acts on theplunger in a lateral direction relative to the axis due to a change inthe acting direction of the pressing force of the cam. When this lateralload acts on the plunger, the plunger swings in the lateral-load actingdirection by an amount corresponding to the backlash (the gap betweenthe male and female threads) of the thread engagement portion and, sincethe plunger moves in the axial-load acting direction while sliding androtating due to this swing of the plunger, the plunger sinks more thanan assumed sinking amount.

Regarding this new problem, if the backlash of the thread engagementportion is made as small as possible so that the influence of thelateral load acting on the plunger can be ignored, i.e., if the backlashis so small that no moment occurs in the thread engagement portion dueto the swing of the plunger, the sinking amount of the plunger in thethread engagement portion becomes proper, and the lash adjusterappropriately operates to eliminate the excessively small state of thevalve clearance. However, it is extremely difficult to perform threadingof the male and female threads constituting the thread engagementportion such that the backlash becomes small, and it is substantiallydifficult to guarantee constant quality of mass-produced lash adjusters.

The present invention was conceived in view of the situations and afirst object thereof is to provide a valve mechanism capable ofautomatically and reliably adjusting a valve clearance.

A second object is to provide a mechanical lash adjuster used in thevalve mechanism.

Means for Solving Problem

To achieve the first object, the following configurations (1) to (6) areemployed.

(1) In a valve mechanism comprising a cam rotating in conjunction withrotation of an engine output shaft; a shaft end portion of a valve urgedin a valve closing direction by a valve spring; a power transmittingmember interposed between the shaft end portion of the valve and the camto transmit a pressing force of the cam to the shaft end portion of thevalve as a valve opening force; and a mechanical lash adjuster linked tothe power transmitting member and adjusting a valve clearance betweenthe cam and the power transmitting member,

the mechanical lash adjuster includes

a plunger that is brought into contact with the power transmittingmember and to which the pressing force of the cam and an urging force ofthe valve spring are transmitted through the power transmitting member,

a plunger engaging member that is put into thread engagement with theplunger to form a thread engagement portion cooperating with the plungerto extend and contract the plunger based on rotation relative to theplunger and that is retained non-rotatably in a circumferentialdirection of the thread engagement portion, and

a compression coil spring associated with the plunger and the plungerengaging member and urging the plunger in a direction in which the powertransmitting member comes into contact with the cam, and

the thread engagement portion is set such that when a load acts on theplunger in one of extension and contraction directions of the plunger,slide rotation of the plunger relative to the plunger engaging member issuppressed in the thread engagement portion by a friction torquegenerated in the thread engagement portion and that when a lateral loadacts on the plunger to cause a swing of the plunger relative to theplunger engaging member, the suppression of the slide rotation isrelieved.

According to this configuration, due to the setting of the threadengagement portion, when the axial load acts on the plunger as a load inone of extension and contraction directions, the thread engagementportion becomes relatively immovable (threads are made self-sustaining),and a drive force according to the rotation of the cam is transmitted tothe power transmitting member. Therefore, the valve can properly beoperated to open and close by utilizing the power transmitting member(when the power transmitting member is a rocker arm, the plungerfunctions as a fulcrum for swinging the rocker arm).

On the other hand, when a lateral load acts on the plunger, the plungeroperates by an amount equivalent to a backlash of the thread engagementportion in the acting direction of the axial load to the plunger (aplunger extension direction (direction of decreasing the valveclearance) or a plunger contraction direction (direction of increasingthe valve clearance)) so that the valve clearance is adjusted, and theadjustment of the valve clearance is implemented by utilizing only theslide rotation of the plunger due to the swinging of the plunger in thelateral-load acting direction based on the backlash without utilizingthe structure causing the plunger to slide and rotate due to the actionof the axial load to the plunger (the structure of Patent Document 3).Therefore, unlike the case that the valve clearance is adjusted by thestructure allowing the axial load to act on the plunger to cause theslide rotation of the plunger, the plunger is prevented from moving morethan an assumed movement amount. Consequently, the valve clearance canautomatically and reliably be adjusted.

Although the lash adjuster is configured such that when the axial loadacts on the plunger in either of extension and contraction directions,the slide rotation of the plunger is suppressed in the thread engagementportion by a friction torque generated in the thread engagement portion,since the lash adjuster is configured to cause the plunger to slide androtate in the thread engagement portion by actively utilizing the factthat the plunger swings due to the lateral load by an amountcorresponding to the backlash of the thread engagement portion, it isnot necessary to make the backlash of the thread engagement portionsmaller than the conventional backlash, and the threading of the maleand female threads constituting the thread engagement portion isaccordingly made easier. Therefore, the present invention is extremelyeffective for mass-production of mechanical lash adjusters with constantquality guaranteed.

(2) Under the configuration of (1),

a torsion spring is associated with the plunger and the plunger engagingmember so that the plunger is urged in a relative rotational directionfor extension from the plunger engaging member.

According to this configuration, even when the structure as describedabove (the valve mechanism described in (1)) is implemented as the valvemechanism and an engine is sequentially cold-started, stopped, andcold-restarted, an abnormal noise can be prevented from occurring basedon collisional contact of the cam with the power transmitting member.

In particular, when the engine is cold-started, the valve becomesextended due to a high-temperature exhaust gas for catalytic activationand the valve clearance is going to be in the excessively small(negative clearance) state and, therefore, to make an adjustment to aproper valve clearance, the plunger deeply enters the plunger engagingmember (the plunger contraction state) and eliminates the excessivelysmall state of the valve clearance.

However, when the engine is stopped in the above state, the state ofsuppressing the slide rotation is maintained in the thread engagementportion and the plunger is retained in the state of having deeplyentered the plunger engaging member, so that when the engine issubsequently restarted while being cold, the valve has contracted andreturned to the original state and, on the other hand, the above state(the state of the plunger having deeply entered the plunger engagingmember) is maintained, and therefore, although the plunger attempts toextend so as to make an adjustment to a proper valve clearance, theplunger cannot extend unless a lateral load acts on the powertransmitting member due to the rotation of the cam, so that the plungermay not promptly return to the properly extended state. Consequently,when the base circle of the cam faces the power transmitting member inthe above case, the clearance between both becomes excessively large,and the cam collisionally comes into contact at an open ramp portionthereof with the power transmitting member and makes an abnormal noise.

Therefore, with the configuration in which a torsion spring isassociated with the plunger and the plunger engaging member so that theplunger is urged in a relative rotational direction for extension fromthe plunger engaging member, the plunger is extended based on the urgingforce of the torsion spring as long as the valve clearance exists and,when the base circle of the cam faces the power transmitting member atthe time of restart, the base circle is always in contact with the powertransmitting member. Consequently, even when the structure as describedabove is implemented as the valve mechanism and the engine issequentially cold-started, stopped, and cold-restarted, the abnormalnoise can be prevented from occurring based on collisional contact ofthe cam with the power transmitting member.

(3) Under the configuration of (2),

the compression coil spring and the torsion spring are constituted as aplunger spring by one spring member.

According to this configuration, while the same effects as (2) describedabove can be implemented, the parts count of the spring membersimplementing the effects can be reduced and the disposition space forarranging the spring members can be made as small as possible.

(4) Under the configuration of (2),

the compression coil spring and the torsion spring are separatelyindependently provided as a plunger spring.

According to this configuration, the compression coil spring and thetorsion spring are individually selected from the viewpoint of thespring coefficient etc., and the springs in the valve mechanism caneasily be adjusted in terms of the urging force.

(5) Under the configuration of (1),

the plunger engaging member is a cylindrical housing retained by acylinder head,

the plunger has one end of the plunger as a contact end for the powertransmitting member and is arranged such that one end side of theplunger projects from the housing while the other end side of theplunger other than the one end side is housed in the housing, and

the thread engagement portion is constituted by a male thread formed onan outer circumferential surface of the plunger and a female threadformed on an inner circumferential surface of the housing and screwedwith the male thread.

According to this configuration, a mechanism having a specific andpreferable structure can be provided as the valve mechanism.

(6) Under the configuration of (1),

the thread engagement portion is set such that due to a lead angle and aflank angle of thread ridges of the threads constituting the threadengagement portion, when a load acts on the plunger in one of extensionand contraction directions of the plunger, slide rotation of the plungerrelative to the plunger engaging member is suppressed in the threadengagement portion by a friction torque generated in the threadengagement portion and that when a lateral load acts on the plunger tocause a swing of the plunger relative to the plunger engaging member,the suppression of the slide rotation is relieved.

According to this configuration, the action of (1) described above canbe specifically be implemented by utilizing the characteristics of thelead angle and the flank angle of the thread ridges of the “threads”constituting the thread engagement portion.

To achieve the second object, the following configurations (7) to (13)are employed.

(7) In a configuration comprising

a plunger;

a plunger engaging member put into thread engagement with the plunger toform a thread engagement portion cooperating with the plunger to extendand contract the plunger based on rotation relative to the plunger; and

a compression coil spring associated with the plunger and the plungerengaging member and urging the plunger in a direction in which theplunger is extended relative to the plunger engaging member,

the thread engagement portion is set such that when a load acts on theplunger in one of extension and contraction directions of the plunger,slide rotation of the plunger relative to the plunger engaging member issuppressed in the thread engagement portion by a friction torquegenerated in the thread engagement portion and that when a lateral loadacts on the plunger to cause a swing of the plunger relative to theplunger engaging member, the suppression of the slide rotation isrelieved.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (1) can be provided.

(8) Under the configuration of (7),

a torsion spring is associated with the plunger and the plunger engagingmember so that the plunger is urged in a relative rotational directionfor extension from the plunger engaging member.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (2) can be provided.

(9) Under the configuration of (8),

the compression coil spring and the torsion spring are constituted as aplunger spring by one spring member.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (3) can be provided.

(10) Under the configuration of (8),

the compression coil spring and the torsion spring are separatelyindependently provided as a plunger spring.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (4) can be provided.

(11) Under the configuration of (7),

the plunger engaging member is a cylindrical housing,

the plunger is arranged such that one end side of the plunger projectsfrom the housing while the other end side of the plunger other than theone end side is housed in the housing, and

the thread engagement portion is constituted by a male thread formed onan outer circumferential surface of the plunger and a female threadformed on an inner circumferential surface of the housing and screwedwith the male thread.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (5) can be provided.

(12) Under the configuration of (7),

the configuration is used in a valve mechanism including a cam rotatingin conjunction with rotation of an engine output shaft, a shaft endportion of a valve urged in a valve closing direction by a valve spring,and a power transmitting member interposed between the shaft end portionof the valve and the cam to transmit a pressing force of the cam to theshaft end portion of the valve as a valve opening force, for adjusting avalve clearance between the cam and the shaft end portion of the valve,

the plunger is brought into contact with the power transmitting memberand arranged such that the pressing force of the cam and an urging forceof the valve spring are transmitted through the power transmittingmember, and

the plunger engaging member is retained non-rotatably in acircumferential direction of the thread engagement portion in the valvemechanism.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (1) can be provided.

(13) Under the configuration of (7),

the thread engagement portion is set such that due to a lead angle and aflank angle of thread ridges of the threads constituting the threadengagement portion, when a load acts on the plunger in one of extensionand contraction directions of the plunger, slide rotation of the plungerrelative to the plunger engaging member is suppressed in the threadengagement portion by a friction torque generated in the threadengagement portion and that when a lateral load acts on the plunger tocause a swing of the plunger relative to the plunger engaging member,the suppression of the slide rotation is relieved.

According to this configuration, a preferable mechanical lash adjusterused in the valve mechanism of (6) can be provided.

Effect of the Invention

As apparent from the above description, according to the valve mechanismof the present invention, the valve clearance can automatically andreliably be adjusted.

According to the mechanical lash adjuster of the present invention, themechanical lash adjuster preferably used in the valve mechanism can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an entire rocker-arm valve mechanismshowing a first embodiment in which the present invention is applied toa mechanical lash adjuster of rocker-arm valve mechanism specifications.

FIG. 2 is a view of a main part of the mechanical lash adjusteraccording to the first embodiment, including (a) a view of lead andflank angles of a thread ridge of a male thread formed on a plunger and(b) a view of lead and flank angles of a thread ridge of a female threadformed on a housing.

FIG. 3 is an explanatory view for explaining a principle of the plungersliding and rotating in a thread engagement portion and moving in anaxial-load acting direction due to swinging of the plunger.

FIG. 4 is diagrams (a) to (d) for explaining motions of the plunger whena lateral load is input to (acts on) an upper end portion of the plungerfrom the near side toward the far side on the plane of the figure,including (a), (b) as the case in which the lateral load acts on theplunger while an axial load acts thereon in an extension direction and(c), (d) as the case in which the lateral load acts on the plunger whilean axial load acts thereon in a contraction direction and showing (a),(c) as diagrams of the plunger viewed from the left with respect to theinput (acting) direction of the lateral load and (b), (d) as diagrams ofthe plunger viewed from the right with respect to the input (acting)direction of the lateral load.

FIG. 5 is a diagram of a valve lift amount, the lateral load acting onthe plunger, and a motion (lift loss) of the plunger when a rotationspeed of an engine is low.

FIG. 6 is a longitudinal sectional view of a mechanical lash adjusterused in the valve mechanism according to the first embodiment.

FIG. 7 is a longitudinal sectional view of a mechanical lash adjusterused in the valve mechanism according to a second embodiment.

FIG. 8 is a perspective view of a torsion spring used in the mechanicallash adjuster according to the second embodiment.

MODES FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described withreference to the drawings.

1. FIGS. 1 to 6 show a first embodiment. In FIG. 1 showing the firstembodiment, reference numeral 1 denotes a valve mechanism of an internalcombustion engine assembled to a cylinder head 11. In this embodiment, arocker-arm valve mechanism is used as the valve mechanism 1, and thevalve mechanism 1 includes as a mechanism opening and closing an intake(exhaust) port P leading to a combustion chamber S, an intake valve oran exhaust valve (hereinafter referred to as a valve) 10 opening/closingthe intake (exhaust) port P, a cam 19 a disposed above the valve 10, arocker arm 16 as a power transmitting member disposed between the valve10 and the cam 19 a, and a mechanical lash adjuster (hereinafterreferred to as the lash adjuster) 20 supporting the rocker arm 16.

(1-1) As is known, the valve 10 integrally includes a stem 10A, and thestem 10A is slidably inserted in a cylindrical valve sliding guide 11 bretained in a through-hole leading to the intake port (or exhaust port)P. The stem 10A has a shaft end portion (in FIG. 1, an upper endportion) projected above an upper surface of the cylinder head 11 with acotter 12 a and a spring retainer 12 b mounted on an outer circumferenceof a tip portion thereof, and a spring seat 11 a is disposed on theupper surface of the cylinder head 11 under the cotter 12 a and thespring retainer 12 b. A valve spring (compression coil spring) 14 iswound around the outer circumference of the stem 10A in a loosely fittedstate, and the valve spring 14 is interposed between the spring retainer12 b and the spring seat 11 a to urge the valve 10 in the direction ofclosing the opening of the intake (exhaust) port P. Reference numeral 10a denotes a taper-shaped seat formed on the outer circumference of ahead of the valve 10, and reference numeral 11 c denotes a seat insertformed into a taper shape corresponding to the seat 10 a on acircumferential edge portion of the opening of the intake (exhaust) portP to the combustion chamber S.

(1-2) The cam 19 a is fixed to a camshaft 19 rotationally driven insynchronization with rotation of an automobile engine. The cam 19 a isrotationally driven according to the rotation of the cam shaft 19. As isknown, the outer circumferential surface of the cam 19 a is made up of abase circle 19 a 1 and a cam nose 19 a 3, and the base circle 19 a 1 andthe cam nose 19 a 3 are divided by an open-side ramp portion 19 a 21 anda close-side ramp portion 19 a 22, and the cam nose 19 a 3 is mostprojected at a cam top 19 a 4.

(1-3) The rocker arm 16 is swung based on the rotational drive of thecam 3. The rocker arm has one end side brought into contact with theshaft end portion of the stem 10A, and a socket portion 18 forsupporting the lash adjuster 20 described later is formed on the otherend side thereof. A roller 17 b supported by a roller shaft 17 a isdisposed in a longitudinal middle of the rocker arm 16 and the cam 19 ais brought into contact with the roller 17 b. This causes the rocker arm16 to swing by using the lash adjuster 20 as a fulcrum based on arotational driving force of the cam 3, and the rotational driving forceof the cam 19 a is transmitted through the swinging of the rocker arm 16to the stem 10A. Consequently, the stem 10A slides on the cylindricalvalve sliding guide 11 b, and the valve 10 opens and closes the intakeport (or the exhaust port) P according to the sliding of the stem 10A.

(1-4) As shown in FIGS. 1 and 6, the lash adjuster 20 has a plunger 24disposed in a cylindrical housing 22 serving as a plunger engagingmember, and a plunger spring 26 is associated with the housing 22 andthe plunger 24.

(1-4-1) The housing 22 is inserted with an opening on one end sidethereof facing upward in a bore 13 formed in the upper side of thecylinder head 11 and extending in the vertical direction. Althoughinserted in the bore 13 such that the other end portion (lower endportion) comes into contact with a bottom surface of the bore 13, thehousing 22 is not press-fitted into the bore 13 (an active housingrotation stopping means is not provided). However, when the plunger 24is pushed down via the rocker arm 16, a friction torque is generatedbetween the other end portion (lower end portion) of the housing 22 andthe bottom surface of the bore 13, and the friction torque prevents thehousing 22 from rotating relative to the bore 13. Therefore, the housing22 is retained so as not to rotate relative to the bore 13 by thefriction torque generated with the bottom surface of the bore 13.

The housing 22A has a female thread 23 formed on an innercircumferential surface thereof, and a disc-shaped spring seat surfaceplate 27 a is accommodated non-rotatably on the other end portion side(lower end portion side) in the housing 22 such that the spring seatsurface plate 27 a is fixed by a C-ring 27 b to the housing 22 andcannot be displaced in the axially direction thereof. A locking hole 31for locking the plunger spring 26 described later is formed in thespring seat surface plate 27 a.

(1-4-2) A rod-shaped member is used for the plunger 24. This plunger 24has a substantially hemispherical pivot portion 24 a formed on one endside and a male thread 25 formed on an outer circumferential surface onthe other end side thereof. The male thread 25 is screwed into thefemale thread 23 on the inner circumferential surface of the housing 22such that one end side of the plunger 24 projects outward from theopening on the one end side of the housing 22, and the pivot portion 24a enters the inside of the socket portion 18 of the rocker arm 16 and isengaged with the socket portion 18. Therefore, axial engagement isachieved between the plunger 24 on which the pressing force of the cam19 a acts as an axial load and the housing 22 retained so as not torotate in a circumferential direction, through the male thread 25 on theplunger 24 side and the female thread 23 on the housing 22 side.

A spring housing hole 32 is formed inside the plunger 24 on the otherend side thereof. The spring housing hole 32 extends in the extensiondirection of the plunger 24 with one end (inner end) of the springaccommodating hole 32 defined by a spring seat surface 33, and a lockinghole 34 for locking the plunger spring 26 described later is formed inthe spring seat surface 33. The other end of the spring housing hole 32is opened to the outside from the other end surface of the plunger 24,and the inside of the spring housing hole 32 faces the spring seatsurface plate 27 a.

(1-4-3) As shown in FIGS. 1 and 6, the plunger spring 26 is interposedbetween the spring seat surface 33 defining the one end of the springhousing hole 32 of the plunger 24 and the spring seat surface plate 27a. In this embodiment, a spring used for the plunger spring 26 has afunction as a torsion spring in addition to the function as acompression coil spring, and the plunger spring 26 is formed into a coilshape at constant pitches such that a wire rod forming the plungerspring 26 is bent at both end portions as locking portions (arms) 35, 36toward the outside in the axial direction of the plunger spring 26. Theplunger spring 26 is housed in the spring housing hole 32 with the axialdirection thereof directed in the extension direction of the springhousing hole 32, and the locking portion 35 on one axial side (lowerside in FIG. 1) is locked in the locking hole 31 of the spring seatsurface plate 27 a while the locking portion 36 on the other axial side(upper side in FIG. 1) is locked in the locking hole 34 of the springseat surface 33. The plunger spring 26 can be twisted and released basedon the relative rotation of the plunger 24 with respect to the housing22 and can be compressed and released (extended) according to extensionand contraction of the housing 22 and the plunger 24 based on therelative rotation of the plunger 24 with respect to the housing 22. In aform in which the base circle 19 a 1 of the cam 19 a is in contact withthe rocker arm 16, the plunger spring 26 urges the plunger 24 in thedirection of extension from the housing 22 due to the function as acompression coil spring and urges the plunger 24 in a relativerotational direction for extension from the housing 22 due to thefunction as a torsion spring.

The spring force of the plunger spring 26 is obviously set weaker thanthe spring force of the valve spring 14.

(1-4-4) The female thread 23 on the housing 22 and the male thread 25 onthe plunger 24 constitute a thread engagement portion 30 through ascrewed relationship between the threads 23, 25. The thread engagementportion 30 is set such that when an axial load acts on the plunger 24 inone of extension and contraction directions, the slide rotation of theplunger 24 relative to the housing 22 is suppressed by the frictiontorque generated in the thread engagement portion 30 to make the threadsself-sustaining (put the thread engagement portion 30 into a relativelyimmovable state) and that when a lateral load acts on the plunger 24,the plunger 24 is allowed to slide and rotate (the suppression of theslide rotation is relieved) and moved in the axial-load actingdirection, and in this embodiment, as shown in enlarged view of FIGS.2(a) and 2(b), the male thread 25 and the female thread 23 are each madeup of a trapezoidal thread such that the thread ridges of the malethread 25 and the female thread 23 are set according to the viewpointdescribed above in terms of a lead angle and a flank angle.

Specifically, the thread angles of the thread ridges of the male thread25 and the female thread 23 are set to a lead angle less than 15 degreesand a flank angle within a range of 5 to 60 degrees. The lead angle isset less than 15 degrees because if the lead angle is 15 degrees or moreand the axial load acts on the plunger 24, the plunger 24 slides androtates in the thread engagement portion 30 and makes it difficult to“reliably make the threads self-sustaining” by the friction torquegenerated in the thread engagement portion 30 and, in contrast, when thelead angle is less than 15 degrees, the plunger 24 having the axial loadacting thereon does not slide and rotate in the thread engagementportion 30 so that the “threads is made self-sustaining” by the frictiontorque generated in the thread engagement portion 30. The flank angle isset within the range of 5 to 60 degrees because if the flank angle isless than 5 degrees, the substantial friction angle of the threadengagement portion 30 falls into a category of a small square thread,which makes changing the flank angle meaningless and highly-accuratemachining without influence of a lead error difficult, and on the otherhand, if the flank angle exceeds 60 degrees, although the “thread” iseasily machined, an extremely large substantial friction angle leads toa considerable influence of lubrication oil and increases a lift lossduring operation of the engine so that the thread cannot practically beused.

More specifically, a lead angle α of the thread ridge of the male thread25 (female thread 23) and an upper flank angle θ25 a (θ23 a) and a lowerflank angle θ25 b (θ23 b) of the thread ridge of the male thread 25(female thread 23) are preferably set to, for example, the lead angleα=10 degrees, the upper flank angle θ25 a, θ23 a=10 degrees, and thelower flank angle θ25 b, θ 23 b=10 degrees.

(1-4-5) Due to such setting of the thread engagement portion 30, whenthe intake (exhaust) port P is operated to open/close by the valve 10,the axial load acts on the plunger 24 via the rocker arm 16 during theoperation in the lash adjuster 20, and the slide rotation of the plunger24 is suppressed by the friction torque generated in the threadengagement portion 30 so that the threads in the thread engagementportion 30 are made self-sustaining and put into an immovable state.Therefore, the pivot portion 24 a at the tip of the plunger 24 functions(acts) as a fulcrum of swinging of the rocker arm 16 swinging incoordination with the rotation of the camshaft 19, and the function asthe fulcrum of swinging of the rocker arm 16 causes the valve 10 toreciprocate in the vertical direction, so that the lift amount of thevalve 10 shows a mound shape in this case as shown in FIG. 5.

When the cam 19 a presses (the roller 17 b of) the rocker arm 16 and theaxial load acts on the plunger 24, the contact point of the cam 19 a 1with (the roller 17 b of) the rocker arm 16 moves on (the roller 17 bof) the rocker arm 16 and changes the acting direction of the pressingforce of the cam 19 a, so that a lateral load also acts on the plunger24 as indicated by reference numerals T1, T2 of FIG. 5. When the lateralload acts on the plunger 24, the plunger 24 swings by an amountcorresponding to the backlash of the thread engagement portion 30relative to the housing 22 in the lateral-load acting direction and, atthe contact point of the male thread 25 with the female thread 23, areaction force based on the lateral load works in the direction along aflank face of the female thread 23. In this case, since the lateral-loadacting direction does not coincide with the direction of the reactionforce at the contact point, the reaction force at the contact point actsas a moment causing the plunger 24 to slide and rotate in the threadengagement portion 30 and the plunger 24 moves in the axial-load actingdirection while sliding and rotating and eliminates a state ofincreased/decreased valve clearance.

2. Description will be made of the principle of the plunger 24 moving inthe axial-load acting direction while sliding and rotating when thelateral load acts on the plunger 24 in more detail with reference toFIGS. 3 and 4.

(2-1) For example, as indicated by reference numeral F1 of FIG. 3, whenthe axial load acting on the plunger 24 is upward (e.g., in a form inwhich only the urging force of the plunger spring 26 acts thereon), anupper flank face 25 a of the male thread 25 is in contact with a lowerflank face 23 b of the female thread 23. A contact point is denoted byreference numeral P1. In FIG. 3, when a lateral load T acts on the pivotportion 24 a at the tip of the plunger 24 arranged in the verticaldirection (see FIG. 1) from the near side toward the far side on theplane of FIG. 3, the plunger 24 uses a lower end portion of the threadengagement portion 30, i.e., a plunger lower end portion 24 b (see FIGS.1 and 4) in thread engagement with the housing-side female thread 23, asa fulcrum such that the pivot portion 24 a at the tip of the plunger 24swings from the near side toward the far side on the plane of FIG. 3.

Therefore, when the thread engagement portion 30 (male thread 25) is anormal right-hand thread, the upper flank face 25 a of the male thread25 in the left half of the male thread 25 (the left half of FIG. 3)operates to push the lower flank face 23 b of the female thread 23extending downward while turning clockwise (on the basis of the downwarddirection), and the upper flank face 25 a of the male thread 25 in theright half of the male thread 25 (the right half of FIG. 3) operates ina direction away from the lower flank face 23 b of the female thread 23extending upward while turning clockwise (on the basis of the upwarddirection).

Thus, the housing-side female thread 23 is retained so as not to rotatein the circumferential direction of the thread engagement portion 30,and therefore, at the contact point P1 of the upper flank face 25 a inthe left half of the male thread 25 with respect to the lower flank face23 b of the female thread 23, a reaction force based on the lateral loadacts in the direction along the lower flank face 23 b of the femalethread 23 extending upward while turning clockwise (on the basis of theupward direction). In this case, since the acting direction (inputdirection) of the lateral load T does not coincide with the direction ofthe reaction force at the contact point P1, the reaction force at thecontact point P1 acts as the moment causing the plunger 24 to slide androtate in the thread engagement portion 30 in a direction R1 of FIG. 3,and the plunger 24 moves in the acting direction of the axial load F1(upward) while sliding and rotating by an amount corresponding to thebacklash.

More specifically, in the left half of the plunger 24 with respect tothe input (acting) direction of the lateral load T, as shown in FIG.4(a), when the plunger 24 swings due to the input of the lateral load,the upper flank face 25 a of the male thread 25 comes into contact withthe lower flank face 23 b of the housing-side female thread 23 retainedso as not to rotate in the circumferential direction and can no longeroperate (move leftward in FIG. 4(a)). On the other hand, in the righthalf of the plunger with respect to the input (acting) direction of thelateral load T, as shown in FIG. 4(b), when the plunger 24 swings due tothe input of the lateral load, the upper flank face 25 a of the malethread 25 moves away from the lower flank face 23 b of the female thread23 (moves rightward in FIG. 4(b)), and is no longer restrained.Consequently, the upper flank face 25 a of the male thread 25 receivesthe reaction force from the lower flank face 23 b of the housing-sidefemale thread 23, and the plunger 24 moves in the extension direction(upward) while sliding and rotating in the direction R1 of FIG. 3 by anamount corresponding to the backlash.

From the above, for example, when the thread engagement portion 30 (malethread 25) is a normal right-hand thread and the axial load F1 acting onthe plunger 24 is upward, the plunger 24 swinging due to the lateralload T moves in the acting direction of the axial load F1 (extensiondirection) while always rotating in the direction R1 of FIG. 3.

(2-2) On the other hand, as indicated by an arrow F2 of FIG. 3, when theaxial load acting on the plunger 24 is downward (e.g., in a form inwhich the urging force of the valve spring 14 acts on the plunger 24 viathe rocker arm 16), a lower flank face 25 b of the male thread 25 is incontact with an upper flank face 23 a of the female thread 23. A contactpoint is denoted by reference numeral P2. When the lateral load T actson the pivot portion 24 a at the tip of the plunger 24 from the nearside toward the far side on the plane of FIG. 3, the plunger 24 uses thelower end portion (plunger lower end portion) 24 b of the threadengagement portion 30 as a fulcrum such that the pivot portion 24 a atthe tip of the plunger 24 swings from the near side toward the far sideon the plane of FIG. 3.

Therefore, when the thread engagement portion 30 (male thread 25) is anormal right-hand thread, the lower flank face 25 b of the male thread25 in the right half of the male thread 25 (the right half of FIG. 3)operates to push the upper flank face 23 a of the female thread 23extending upward while turning clockwise (on the basis of the upwarddirection), and the lower flank face 25 b of the male thread 25 in theleft half of the male thread 25 (the left half of FIG. 3) operates in adirection away from the upper flank face 23 a of the female thread 23extending downward while turning clockwise (on the basis of the downwarddirection).

Thus, the housing-side female thread 23 is retained so as not to rotatein the circumferential direction of the thread engagement portion 30,and therefore, at the contact point P2 of the lower flank face 25 b inthe right half of the plunger-side male thread 25 with respect to theupper flank face 23 a of the housing-side female thread 23, a reactionforce based on the lateral load acts in the direction along the upperflank face 23 a of the female thread 23 extending downward while turningclockwise (on the basis of the downward direction). In this case, sincethe acting direction of the lateral load T does not coincide with thedirection of the reaction force at the contact point P2, the reactionforce at the contact point P2 acts as the moment causing the plunger 24to slide and rotate in the thread engagement portion 30 in a directionR2 of FIG. 3, and the plunger 24 moves in the acting direction of theaxial load F2 (downward) while sliding and rotating by an amountcorresponding to the backlash.

More specifically, in the right half of the plunger 24 with respect tothe input (acting) direction of the lateral load T, as shown in FIG.4(d), when the plunger 24 swings due to the lateral load T, the lowerflank face 25 b of the male thread 25 comes into contact with the upperflank face 23 a of the female thread 23 and can no longer operate (moverightward in FIG. 4(d)). On the other hand, in the left half of theplunger 24 with respect to the input (acting) direction of the lateralload T, as shown in FIG. 4(c), when the plunger 24 swings due to thelateral load T, the lower flank face 25 b of the male thread 25 movesaway from the upper flank face 23 a of the female thread 23 and is nolonger restrained (moving leftward in FIG. 4(c)). Consequently, thelower flank face 25 b of the male thread 25 receives the reaction forcefrom the upper flank face 23 a of the housing-side female thread 23, andthe plunger 24 moves in the contraction direction (downward) whilesliding and rotating in the direction R2 of FIG. 3 by an amountcorresponding to the backlash.

From the above, for example, when the thread engagement portion 30 (malethread 25) is a normal right-hand thread and the axial load F2 acting onthe plunger 24 is downward, the plunger 24 swinging due to the lateralload T moves in the acting direction of the axial load F2 (contractiondirection) while always rotating in the direction R2 of FIG. 3.

(2-3) As described above, when the lead angle and the flank angle of thethread ridges of the “threads” constituting the thread engagementportion 30 are set to predetermined values (e.g., the lead angle α=10degrees, the upper flank angle θ25 a, θ 23 a=10 degrees, and the lowerflank angle θ25 b, θ 23 b=10 degrees), the plunger 24 having the axialload acting thereon basically becomes relatively immovable in the threadengagement portion 30 (the threads are made self-sustaining) andfunctions (acts) as a fulcrum of swinging of the rocker arm 16, so thatwhen the lateral load T acts on the plunger 24, the plunger 24 operatesby an amount corresponding to the backlash of the thread engagementportion 30 not only in the extension direction of the plunger 24 (thedirection of decreasing the valve clearance) but also in the contractiondirection of the plunger 24 (the direction of increasing the valveclearance).

3. An operation of a valve mechanism incorporating the lash adjuster 20will be described.

(3-1) As shown in FIGS. 1 and 5, when the cam shaft 19 (the cam 19 a)rotates, the contact point of the cam 19 a with (the roller 17 b of) therocker arm 16 is on the cam nose 19 a 3 at a cam angle from about −60degrees to about +60 degrees and is on the base circle 19 a 1 of the cam19 a at the cum angle in the other range (about −60 degrees or less andabout +60 degrees or more). While the cam angle is about −60 degrees toabout +60 degrees, the contact point of the cam 19 a with the rocker arm16 is located on one side surface of the cam nose 19 a 3 from theopen-side ramp portion 19 a 21 to the cam top 19 a 4 of the cam at thecam angle from about −60 degrees to 0 degrees, and the contact point ofthe cam 19 a with the rocker arm 16 is located on the other side surfaceof the cam nose 19 a 3 from the cam top 19 a 4 to the close-side rampportion 19 a 22 at the cam angle from 0 degrees to about +60 degrees.

(3-2) First, when the contact point of the cam 19 a with the rocker arm16 is on the base circle 19 a 1 of the cam 19 a (when the cam angle is−60 degrees or less), a predetermined urging force of the plunger spring26 acts on the plunger 24, and this urging force is balanced with thefriction force generated on the thread engagement portion 30 (threadsurfaces) so that the plunger 24 does not move in theextension/contraction direction with the valve clearance (clearancebetween the cam 19 a and the rocker arm 16) retained at zero. Therefore,the plunger 24 becomes immovable with “threads made self-sustaining” inthe thread engagement portion 30, and the lash adjuster 20 functions asthe fulcrum of swinging of the rocker arm 16.

(3-3) When the contact point of the cam 19 a with the rocker arm 16 islocated between the open-side ramp portion 19 a 21 of the cam and theclose-side ramp portion 19 a 22 on the opposite side across the cam top19 a 4 (when the cam angle of FIG. 5 is in the range from −60 degrees to+60 degrees), the pressing force due to the cam 19 a acts as an axialload on the plunger 24 via the rocker arm 16. Therefore, the plunger 24becomes immovable with “threads made self-sustaining” in the threadengagement portion 30 so that the lash adjuster 20 functions as thefulcrum of swinging of the rocker arm 16, and a lift amount of the valve10 corresponding to one rotation of the cam 19 a forms a mound shapewith a Max lift of about 10 mm as indicated by a broken line of FIG. 5.Although described in detailed later, because of a backlash present inthe thread engagement portion 30 between the plunger 24 and the housing22, the lift amount of the valve 10 shown in FIG. 5 includes a lift lossδ (e.g., about 0.2 mm) generated as the plunger 24 automatically slidesand rotates to move in the contraction direction.

(3-4) When the pressing force from the cam 19 a acts as the axial loadon the plunger 24 via the rocker arm 16, since the contact point of thecam 19 a with (the roller 17 b) of the rocker arm 16 moves according tothe rotary movement of the cam 19 a and the acting direction of thepressing force on (the roller 17 b of) the rocker arm 16 of the cam 19 achanges, the lateral loads T1, T2 of about 250 to 150 N act on theplunger 24 as shown in FIG. 5. The valve mechanism 1 uses the lateralloads T1, T2 to adjust the positive (negative) valve clearance generatedin the valve mechanism 1.

(3-4-1) When the contact point of the cam 19 a with the rocker arm 16shifts from the open-side ramp portion 19 a 21 to the cam nose 19 a 3,the positive (negative) valve clearance generated in the valve mechanism1 is adjusted as follows.

(3-4-1-1) The positive valve clearance in the valve mechanism 1 ismanifested as a gap between the cam 19 a and the roller 17 b of therocker arm 16 when the contact point of the cam 19 a with the rocker arm16 is on the base circle 19 a 1 of the cam 19 a. In this case, theurging force of the plunger spring 26 acts on the plunger 24, and thisurging force is balanced with the friction force generated on the threadengagement portion 30 (thread surfaces) so that the threads of thethread engagement portion 30 are retained in the self-sustaining state.

In this state, when the contact point (contact point with a gap) of thecam 19 a with the rocker arm 16 shifts from the open-side ramp portion19 a 21 to the cam nose 19 a 3, the lateral load T1 (see FIG. 5) acts onthe plunger 24 according to the shift of the contact point. This lateralload T1 acts via the rocker arm 16 on the plunger 24 in the immovablestate in which the axial load acts in the extension direction due to theurging force of the plunger spring 26 immediately before the pressingforce of the cam 19 a acts as the axial load and, based on this action,the plunger 24 moves in the extension direction that is the axial-loadacting direction. As a result, the plunger 24 pushes up the rocker arm16 while sliding and rotating, and the positive clearance generated inthe valve mechanism 1 is adjusted to zero.

Specifically, when the lateral load T1 (see FIG. 5) acts on the plunger24 via the rocker arm 16, the plunger 24 swings by an amountcorresponding to the backlash in the thread engagement portion 30between the female thread 23 and the male thread 25 in the actingdirection of the lateral load T1 on the lower end portion 24 b of theplunger 24 used as the fulcrum, and the reaction force based on thelateral load works in the direction along the lower flank face 23 b ofthe female thread 23 at the contact point P1 (see FIG. 3) of the malethread 25 with the female thread 23. The reaction force at the contactpoint P1 acts as the moment causing the plunger 24 to slide and rotatein the thread engagement portion 30 and the plunger 24 moves in theaxial-load acting direction (the acting direction of the urging force ofthe plunger spring 26, the plunger extension direction) while slidingand rotating, and adjusts the positive valve clearance to zero.

(3-4-1-2) On the other hand, the negative valve clearance in the valvemechanism 1 is manifested as an excessively small gap (negative gap)between the cam 19 a and the roller 17 b since the rocker arm 16 (theroller 17 b) is pressed by the base circle 19 a 1 of the cam 19 a due tothe urging force of the valve spring 14 when the contact point of thecam 19 a with the rocker arm 16 is on the base circle 19 a 1 of the cam19 a. In this case, although the urging force of the valve spring 14acts on the plunger 24 via the rocker arm 16 as the axial load in thecontraction direction, this urging force is balanced with the frictionforce generated on the thread engagement portion 30 (thread surfaces) sothat the threads of the thread engagement portion 30 are retained in theself-sustaining state.

In this state, when the contact point (negative gap) of the cam 19 awith the rocker arm 16 shifts from the open-side ramp portion 19 a 21 tothe cam nose 19 a 3, the lateral load T1 acts on the plunger 24according to the shift of the contact point. This lateral load T1 actsvia the rocker arm 16 on the plunger 24 in the immovable state in whichonly the urging force of the valve spring 14 acts as the axial loadimmediately before the pressing force of the cam 19 a acts as the axialload and, based on this action, the plunger 24 moves in the contractiondirection that is the axial-load acting direction while sliding androtating. As a result, the cam 19 a pushes down the rocker arm 16, andthe negative clearance generated in the valve mechanism 1 is adjusted tozero.

Specifically, when the lateral load T1 (see FIG. 5) acts on the plunger24 via the rocker arm 16, the plunger 24 swings by an amountcorresponding to the backlash in the thread engagement portion 30between the female thread 23 and the male thread 25 in the actingdirection of the lateral load T1 on the lower end portion 24 b used asthe fulcrum, and the reaction force based on the lateral load works inthe direction along the upper flank face 23 a of the female thread 23 atthe contact point P2 (see FIG. 3) of the male thread 25 with the femalethread 23. The reaction force at the contact point P2 acts as the momentcausing the plunger 24 to slide and rotate in the thread engagementportion 30 and the plunger 24 moves in the plunger contraction directionthat is the acting direction of the axial load (the urging force of thevalve spring 14) while sliding and rotating, and adjusts the valveclearance to zero.

(3-4-2) When the contact point of the cam 19 a with the rocker arm 16shifts from the cam nose 19 a 3 to the close-side ramp portion 19 a 22,the positive (negative) valve clearance generated in the valve mechanism1 is adjusted as follows.

(3-4-2-1) First, description will be made of the case that the lateralload T2 of FIG. 5 acts on the plunger 24 when the positive valveclearance exists on the base circle 19 a 1 of the cam 19 a.

When the contact point of the cam 19 a with the rocker arm 16 (thecontact point with an inherent gap) shifts from the cam nose 19 a 3 tothe close-side ramp portion 19 a 22, the lateral load T2 acts on theplunger 24 according to the shift of the contact point. Specifically, asthe cam 19 a rotates, the pressing force of the cam 19 a against therocker arm 16 becomes weaker when the contact point of the cam 19 a withthe roller 17 b comes closer to the close-side ramp 19 a 2 of the cam 19a, and a gap is generated between the cam 19 a and the roller 17 b (theinherent gap in the contact point is manifested) before the contactpoint shifts to the close-side ramp 19 a 2. In this state in which thepressing force of the cam 19 a against the rocker arm 16 becomes weakerand the axial load acting on the plunger 24 (the reaction force of thevalve spring 14) is almost eliminated immediately before the gap isgenerated (manifested), the lateral load T2 (see FIG. 5) acts on theplunger 24 according to the shift of the contact point of the cam 19 awith the rocker arm 16. Therefore, the lateral load T2 (see FIG. 5) actsvia the rocker arm 16 on the plunger 24 on which the axial load acts inthe extension direction due to the urging force of the plunger spring26, and the plunger 24 moves in the extension direction that is theaxial-load acting direction. As a result, the plunger 24 pushes up therocker arm 16 and the positive valve clearance on the base circle 19 a 1of the cam 19 a (positive valve clearance generated in the valvemechanism 1) is adjusted to zero.

(3-4-2-2) On the other hand, the negative valve clearance in the valvemechanism 1 is manifested as a form in which a gap is generated betweenthe seat 10 a and the seat insert 11 c of the valve 10 when the valve 10is in the state of closing the intake (exhaust) port P, i.e., when thecontact point of the cam 19 a with the rocker arm 16 is on the basecircle 19 a 1 of the cam 19 a. In this state, since the roller 17 b ofthe rocker arm 16 is pressed against the cam 19 a by the urging force ofthe valve spring 14, the urging force of the valve spring 14 acts on theplunger 24 of the lash adjuster 20 via the rocker arm 16 as the axialload in the contraction direction.

Therefore, when the lateral load T2 (see FIG. 5) acts via the rocker arm16 on the plunger 24 on which the urging force of the valve spring 14acts as the axial load in the contraction direction as the pressingforce of the cam decreases immediately before the contact point of thecam 19 a with the rocker arm 16 shifts from the cam nose 19 a 3 to theclose-side ramp portion 19 a 22, the plunger 24 moves in the contractiondirection that is the axial-load acting direction, and the cam 19 apushes down the locker arm 16, so that the negative valve clearancegenerated in the valve mechanism 1 is adjusted to zero.

(3-4-2-3) For example, if an engine is rapidly cooled after beingstopped in a warmed state, the valve clearance may be put into anexcessively small (negative) state due to a difference in thermalexpansion coefficient between a cylinder head (aluminum alloy) and avalve (iron alloy) so that a face surface of the valve may float from avalve seat. If the valve seat surface is worn out, the same thinghappens (the valve clearance is put into the excessively small state andthe face surface of the valve floats from the valve seat). If the engineis started and driven in such an excessively small (negative) state ofthe valve clearance, the combustion chamber is not sealed and anappropriate output cannot be acquired.

However, in this embodiment, in the excessively small state of the valveclearance, the lateral load acts via the rocker arm 16 on theself-sustaining plunger 24 on which only the urging force of the valvespring 14 acts as the axial load immediately after the start of the liftof the valve or immediately before the end of the lift and, when theplunger 24 swings in the lateral-load acting direction, the reactionforce works at the contact point P2 in the thread engagement portion 30so that the moment is generated. Consequently, the plunger 24 moves inthe plunger contraction direction that is the axial-load actingdirection, i.e., in the direction of increasing the valve clearance,while sliding and rotating in the thread engagement portion 30, and theexcessively small state of the valve clearance is eliminated.

Therefore, when the engine is driven, the combustion chamber canreliably be sealed by the valve 10 and an appropriate output can beacquired.

(3-5) Therefore, when the axial load acts on the plunger 24 as a load inone of the extension and contraction directions thereof in the valvemechanism 1 of this embodiment, the thread engagement portion 30 can bemade relatively immovable (the threads are made self-sustaining) and theplunger 24 can be allowed to function as a fulcrum for swinging therocker arm 16 and, on the other hand, when the lateral load acts on theplunger 24, the plunger 24 operates by an amount equivalent to thebacklash of the thread engagement portion 30 in the directioncorresponding to the acting direction of the axial load to the plunger24 (the plunger extension direction (the direction of decreasing thevalve clearance) or the plunger contraction direction (the direction ofincreasing the valve clearance)) so that the valve clearance isadjusted, and the adjustment of the valve clearance is implemented byutilizing only the slide rotation of the plunger 24 due to the swingingof the plunger 24 in the lateral-load acting direction based on thebacklash without utilizing the structure causing the plunger 24 to slideand rotate due to the action of the axial load to the plunger 24 (thestructure of Patent Document 3). Therefore, unlike the case that thevalve clearance is adjusted by the structure allowing the axial load toact on the plunger 24 to cause the slide rotation of the plunger 24, theplunger is prevented from moving more than an assumed movement amountand the valve clearance can automatically and reliably be adjusted.

4. Since the backlash is present in the thread engagement portion 30between the plunger 24 and the housing 22 of the lash adjuster 20, whenthe valve 10 performs a descending operation in coordination with therotation of the cam 19 a, the plunger 24 automatically moves in thecontraction direction to reduce the lift amount so that the lift loss δis generated; however, the lift loss δ is automatically eliminated by acorrection function of the lash adjuster 20.

In particular, when the contact point of the cam 19 a with the rockerarm 16 shifts from the open-side ramp portion 19 a 21 to the cam nose 19a 3 of the cam 19 a, both the axial load and the lateral load always acton the lash adjuster 20 as shown in FIGS. 1, 3, 4, and 5. When thelateral load T1 (see FIG. 5) acts on the plunger 24, the direction ofmovement thereof is determined by the axial-load acting direction.Specifically, when the contact point of the cam 19 a is on the basecircle 19 al of the cam 19 a (when the cam angle is less than −60degrees), the urging force of the plunger spring 26 acts on the plunger24, and the friction force balanced with this urging force is generatedon the thread surfaces of the thread engagement portion 30. Therefore,the plunger 24 is retained in the immovable state without moving in theextension/contraction direction, and the valve clearance (the gapbetween the cam 19 a and the rocker arm 16) is maintained at zero.

Subsequently, when the contact point of the cam 19 a shifts from thebase circle 19 a 1 to the open-side ramp portion 19 a 21, a set load(the pressing force of the cam 19 a, i.e., the urging force of the valvespring 14) F2 of the valve 10 suddenly acts on the plunger 24 as theaxial load.

When the lateral load denoted by T1 in FIG. 5 acts on the plunger 24 viathe rocker arm 16 while the axial load F2 in the contraction directionacts on the plunger 24, the plunger 24 swinging in the acting directionof the lateral load T1 slides and rotates in the thread engagementportion 30 so as to move in the contraction direction (upward in FIG.5). Therefore, the socket portion 18 of the rocker arm 16 descends (theother end side of the rocker arm 16 ascends) by an amount correspondingto the movement amount of the plunger 24 in the contraction directionand the lift amount of the valve 10 decreases, resulting in the liftloss δ (see FIG. 5).

After this lift loss δ is generated, the plunger 24 can no longer swingand, therefore, the lift amount of the valve 10 gradually increasesuntil the contact point of the cam 19 a shifts to the top 19 a 4 of thecam nose 19 a 3; however, the lash adjuster 20 is retained in thecontracted state and the lift loss δ is maintained as it is. While thecam 19 a rotates and the lift amount of the valve 10 gradually decreasesfrom the Max lift, the lateral load T2 (see FIG. 5) in the directionopposite to the lateral load T1 acts on the plunger 24 via the rockerarm 16; however, since the pressing force of the cam 19 a (the urgingforce of the valve spring 14) is dominant in the axial load acting onthe plunger 24, the lash adjuster 20 is kept in the contracted stateregardless of the action of the lateral load T2. In other words, thevalue of the lateral load acting on the plunger is extremely small(almost no lateral load acts) in the vicinity of the Max lift, while thepressing force of the cam 19 a (the urging force of the valve spring 14)is close to the maximum value, and therefore, the plunger 24 does notswing/slide or rotate so that the lash adjuster 20 is retained in thecontracted state.

However, when the contact point of the cam 19 a shifts to the close-sideramp portion 19 a 22 of the cam 19 a, the axial load acting on theplunger 24 (the pressing force of the cam 19 a, i.e., the urging forceof the valve spring 14) decreases, and the urging force from the plungerspring 26 acts as the axial load F1. When the lateral load T2 acts viathe rocker arm 16 in a state in which the direction of action of theaxial load changes in this way, i.e., when the lateral load T2 acts onthe plunger 24 on which the urging force by the plunger spring 26 actsas the axial load F1, as shown in FIGS. 4(a) and 4(b), the plunger 24having been in the contracted state until then swings/slides and movesin the acting direction of the axial load F1 (the extension direction)and the lift loss δ disappears.

Therefore, in this embodiment, since the backlash is present in thethread engagement portion 30 between the plunger 24 and the housing 22of the lash adjuster 20, the contact point between the rocker arm 16 andthe cam 19 a shifts from the open-side ramp portion 19 a 21 to the camnose 19 a 3 of the cam 19 a, the lift loss δ is generated, and when thecontact point between the rocker arm 16 and the cam 19 a shifts from thecam nose 19 a 3 to the close-side ramp portion 19 a 22, the lift loss δautomatically disappears.

As described above, since the valve clearance automatic adjustmentfunction of the lash adjuster 20 allows the lash adjuster 20 to contractand extend according to the input variation of one cam rotation, thelift loss δ is always generated in the valve mechanism 1. Conversely, itis shown that if the lift loss δ is generated in the valve mechanism 1during the normal operation of the engine, the lash adjuster 20 cancorrect the positive/negative variation in the valve clearanceencountered during the operation of the engine.

5. Also in this embodiment, for example, even when the engine issequentially cold-started, stopped, and cold-restarted under the valvemechanism structure as described above, the valve clearance adjustmentis properly performed before the restart and, when the base circle ofthe cam 19 a faces the rocker arm 16 at the time of the restart, thebase circle is always in contact with the rocker arm 16.

(5-1) Description will be made in detail. When the engine iscold-started, the valve becomes extended due to a high-temperatureexhaust gas for catalytic activation and the valve clearance is going tobe in the excessively small (negative clearance) state and, therefore,to make an adjustment to a proper valve clearance, the plunger 24 deeplyenters the inside of the housing 22 (the plunger contraction state) andeliminates the excessively small state of the valve clearance.

However, when the engine is stopped in the above state, the state ofsuppressing the slide rotation is maintained in the thread engagementportion 30 and the plunger 24 is retained in the state of having deeplyentered the housing 22, so that when the engine is subsequentlyrestarted while being cold, the valve 10 has contracted and returned tothe original state and, on the other hand, the above state (the state ofthe plunger 24 having deeply entered the housing 22) is maintained.Therefore, although the plunger 24 attempts to extend so as to make anadjustment to a proper valve clearance, the plunger 24 cannot extendunless a lateral load acts on the rocker arm 16 due to the rotation ofthe cam 19 a, so that the plunger 24 may not promptly return to theproperly extended state. Consequently, when the base circle of the cam19 a faces the rocker arm 16 before the plunger 24 returns to theproperly extended state, the clearance between both 16, 19 a is in anexcessively large state, and the cam 19 a collisionally comes intocontact at an open ramp portion thereof with the rocker arm 16 and makesan abnormal noise.

(5-2) Therefore, a spring having a function as a torsion spring is usedas the plunger spring 26 in this embodiment in consideration of theproblem described above and, based on the function as a torsion spring,the plunger 24 is always urged in the direction in which the plunger isextended due to rotation relative to the housing 22. As a result, theplunger 24 is relatively rotated and extended by the urging force basedon the function as a torsion spring in the plunger spring 24 as long asthe valve clearance exists and, when the base circle of the cam 19 afaces the rocker arm 16 at the time of restart, the base circle isalways in contact with the rocker arm 16. Consequently, even when theengine is sequentially cold-started, stopped, and cold-restarted underthe structure as described above serving as the valve mechanism, theabnormal noise can be prevented from occurring due to collisionalcontact of the cam 19 a with the rocker arm 16.

(5-3) Moreover, since the plunger spring 26 is made up of one springmember to serve as both a compression coil spring and a torsion springin this embodiment, the parts count of necessary spring members can bereduced and the disposition space for arranging the spring members canbe made as small as possible. Therefore, a preferable member can beprovided as the spring material housed in the narrow spring housing hole32 in the plunger 24.

6. FIGS. 7 and 8 show a second embodiment. In the second embodiment, thesame constituent elements as those of the first embodiment are denotedby the same reference numerals and will not be described.

(6-1) The second embodiment shown in FIGS. 7 and 8 shows a modificationexample of the first embodiment. In the second embodiment, a compressioncoil spring 26 a and a torsion spring 26 b are separately independentlyprovided as the plunger spring 26.

An ordinary spring is used as the compression coil spring 26 a, and thecompression coil spring 26 a is interposed between the spring seatsurface 33 defining one end of the spring housing hole 32 of the plunger24 and the spring seat surface plate 27 a so as to urge the plunger 24in a direction of extension from the housing 22.

As shown in FIG. 8, a spring used as the torsion spring 26 b is formedinto a contact coil spring shape such that a wire rod forming the springis bent at both end portions as the locking portions (arms) 35, 36toward the outside in the axial direction thereof. The torsion spring 26b is disposed on the spring seat surface plate 27 a on the outercircumferential side of the compression coil spring 26 a with the axialdirection thereof directed in the axial direction of the plunger 24, andthe locking portion 35 on one axial side (lower side in FIG. 1) of theplunger spring 26 is locked in the locking hole 31 of the spring seatsurface plate 27 a while the locking portion 36 on the other axial side(upper side in FIG. 1) is locked to the plunger 24.

In this case, since the spring having the contact coil spring shape isused as the torsion spring 26 b and, on the other hand, theextension/contraction motion (stroke) of the plunger 24 must be ensuredwhile maintaining the torsion spring force thereof, a locking groove 37extending relatively long in the axial direction of the plunger 24 isformed in the plunger 24 to lock the locking portion 36 in the lockinggroove 37, and the locking portion 37 extends relatively long in theextension/contraction direction of the plunger 24 while maintaining alocking relationship with the locking groove 37. Similar to the firstembodiment, the torsion spring 26 b urges the plunger 24 in the relativerotational direction for extension from the housing 22. Obviously, avalue acquired by converting the spring coefficient of the torsionspring 26 b into an axial load through the thread engagement portion 30is smaller than the spring coefficient of the valve spring 14.

(6-2) Also in the second embodiment, in addition to the same effects asthe first embodiment, the compression coil spring 26 a and the torsionspring 26 b can individually be selected from the viewpoint of thespring coefficient etc., and the springs in the valve mechanism 1 caneasily be adjusted in terms of the urging force.

7. Although the embodiments have been described, the present inventionincludes the following forms:

(i) applying the valve mechanism or the mechanical lash adjusteraccording to the present invention to the direct-acting valve mechanism(FIGS. 6 and 7) and the rocker-arm valve mechanism (FIG. 8) described inthe international application (PCT/2016/068045) serving as a basis ofpriority claim;

(ii) using the male thread 25 and the female thread 23 made up oftriangular threads;

(iii) using the male thread 25 and the female thread 23 made up oftrapezoidal threads or triangular threads having unequal flank angles,i.e., upper and lower flank angles different from each other;

(iv) using the male thread 25 and the female thread 23 made up ofmultiple threads such as those of two-threaded screws and three-threadedscrews having multiple leads; and (v) configuring the backlash of thethread engagement portion 30 to change continuously or stepwise in theaxial direction of the plunger 24.

EXPLANATIONS OF LETTERS OR NUMERALS

-   10 valve-   11 cylinder head-   14 valve spring-   16 rocker arm (power transmitting member)-   19 a cam-   20 mechanical lash adjuster-   22 housing (plunger engaging member)-   23 female thread-   24 plunger-   25 male thread-   26 plunger spring-   26 a compression coil spring (plunger spring)-   26 b torsion spring (plunger spring)-   F1, F2 axial load acting on the plunger-   T, T1, T2 lateral load acting on the plunger-   α lead angle of a thread ridge-   θ23 a upper flank angle of the thread ridge of the female thread-   θ23 b lower flank angle of the thread ridge of the female thread-   θ25 a upper flank angle of the thread ridge of the male thread-   θ25 b lower flank angle of the thread ridge of the male thread

1. A valve mechanism comprising: a cam rotating in conjunction with rotation of an engine output shaft; a shaft end portion of a valve urged in a valve closing direction by a valve spring; a power transmitting member interposed between the shaft end portion of the valve and the cam to transmit a pressing force of the cam to the shaft end portion of the valve as a valve opening force; and a mechanical lash adjuster linked to the power transmitting member and adjusting a valve clearance between the cam and the power transmitting member, wherein the mechanical lash adjuster includes a plunger that is brought into contact with the power transmitting member and to which the pressing force of the cam and an urging force of the valve spring are transmitted through the power transmitting member, a plunger engaging member that is put into thread engagement with the plunger to form a thread engagement portion cooperating with the plunger to extend and contract the plunger based on rotation relative to the plunger and that is retained non-rotatably in a circumferential direction of the thread engagement portion, and a compression coil spring associated with the plunger and the plunger engaging member and urging the plunger in a direction in which the power transmitting member comes into contact with the cam, and wherein the thread engagement portion is set such that when a load acts on the plunger in one of extension and contraction directions of the plunger, slide rotation of the plunger relative to the plunger engaging member is suppressed in the thread engagement portion by a friction torque generated in the thread engagement portion and that when a lateral load acts on the plunger to cause a swing of the plunger relative to the plunger engaging member, the suppression of the slide rotation is relieved.
 2. The valve mechanism according to claim 1, wherein a torsion spring is associated with the plunger and the plunger engaging member so that the plunger is urged in a relative rotational direction for extension from the plunger engaging member.
 3. The valve mechanism according to claim 2, wherein the compression coil spring and the torsion spring are constituted as a plunger spring by one spring member.
 4. The valve mechanism according to claim 2, wherein the compression coil spring and the torsion spring are separately independently provided as a plunger spring.
 5. The valve mechanism according to claim 1, wherein, the plunger engaging member is a cylindrical housing retained by a cylinder head, wherein the plunger has one end of the plunger as a contact end for the power transmitting member and is arranged such that one end side of the plunger projects from the housing while the other end side of the plunger other than the one end side is housed in the housing, and wherein the thread engagement portion is constituted by a male thread formed on an outer circumferential surface of the plunger and a female thread formed on an inner circumferential surface of the housing and screwed with the male thread.
 6. The valve mechanism according to claim 1, wherein, the thread engagement portion is set such that due to a lead angle and a flank angle of thread ridges of the threads constituting the thread engagement portion, when a load acts on the plunger in one of extension and contraction directions of the plunger, slide rotation of the plunger relative to the plunger engaging member is suppressed in the thread engagement portion by a friction torque generated in the thread engagement portion and that when a lateral load acts on the plunger to cause a swing of the plunger relative to the plunger engaging member, the suppression of the slide rotation is relieved.
 7. A mechanical lash adjuster comprising: a plunger; a plunger engaging member put into thread engagement with the plunger to form a thread engagement portion cooperating with the plunger to extend and contract the plunger based on rotation relative to the plunger; and a compression coil spring associated with the plunger and the plunger engaging member and urging the plunger in a direction in which the plunger is extended relative to the plunger engaging member, wherein the thread engagement portion is set such that when a load acts on the plunger in one of extension and contraction directions of the plunger, slide rotation of the plunger relative to the plunger engaging member is suppressed in the thread engagement portion by a friction torque generated in the thread engagement portion and that when a lateral load acts on the plunger to cause a swing of the plunger relative to the plunger engaging member, the suppression of the slide rotation is relieved.
 8. The mechanical lash adjuster according to claim 7, wherein a torsion spring is associated with the plunger and the plunger engaging member so that the plunger is urged in a relative rotational direction for extension from the plunger engaging member.
 9. The mechanical lash adjuster according to claim 8, wherein the compression coil spring and the torsion spring are constituted as a plunger spring by one spring member.
 10. The mechanical lash adjuster according to claim 8, wherein the compression coil spring and the torsion spring are separately independently provided as a plunger spring.
 11. The mechanical lash adjuster according to claim 7, wherein the plunger engaging member is a cylindrical housing, wherein the plunger is arranged such that one end side of the plunger projects from the housing while the other end side of the plunger other than the one end side is housed in the housing, and wherein the thread engagement portion is constituted by a male thread formed on an outer circumferential surface of the plunger and a female thread formed on an inner circumferential surface of the housing and screwed with the male thread.
 12. The mechanical lash adjuster according to claim 7, wherein the mechanical lash adjuster is used in a valve mechanism including a cam rotating in conjunction with rotation of an engine output shaft, a shaft end portion of a valve urged in a valve closing direction by a valve spring, and a power transmitting member interposed between the shaft end portion of the valve and the cam to transmit a pressing force of the cam to the shaft end portion of the valve as a valve opening force, for adjusting a valve clearance between the cam and the shaft end portion of the valve, wherein the plunger is brought into contact with the power transmitting member and arranged such that the pressing force of the cam and an urging force of the valve spring are transmitted through the power transmitting member, and wherein the plunger engaging member is retained non-rotatably in a circumferential direction of the thread engagement portion in the valve mechanism.
 13. The mechanical lash adjuster according to claim 7, wherein the thread engagement portion is set such that due to a lead angle and a flank angle of thread ridges of the threads constituting the thread engagement portion, when a load acts on the plunger in one of extension and contraction directions of the plunger, slide rotation of the plunger relative to the plunger engaging member is suppressed in the thread engagement portion by a friction torque generated in the thread engagement portion and that when a lateral load acts on the plunger to cause a swing of the plunger relative to the plunger engaging member, the suppression of the slide rotation is relieved. 